In recent years, there is used a waterproof camera having the structure that stands up to use in surroundings where the camera is exposed to wind and rain as well as the open air is used as a car-mounted camera and an outdoor type camera.
In the waterproof camera of that kind, there is known a technology to incorporate a lens representing an optical element in a case body through a rubber gasket such as an O-ring, for providing a waterproof function that prevents a drop of water from leaking into the camera.
For example, FIG. 2 and FIGS. 4-7 in Unexamined Japanese Patent Application Publication (JP-A) No. 2002-90603 disclose a technology to incorporate a lens in a case body with an O-ring arranged on the rear side of the lens. In this case, the O-ring is compressed in the optical axis direction because the lens is pushed into the case body from the front side of the case body to be mounted. An arrangement of the lens is sometimes shifted in the optical axis direction, because repulsive power of the O-ring pushes back the lens in the optical axis direction.
If the lens position in the camera is shifted in the optical axis direction, it causes a trouble that an image of a photographic subject cannot be formed properly on an image pickup device such as CCD. In particular, if the lens is shifted in the optical axis direction in a wide-angle lens, this causes a critical problem.
FIG. 3 in JP-A No. 2002-90603 also discloses a technology that a stepped portion is formed on the circumference surface of the lens and an O-ring is arranged on the stepped portion. By incorporating the lens into the case body with the O-ring pressed in a direction perpendicular to an optical axis of the lens, the shift of the lens in the optical axis direction, which is cause due to the above arrangement of the O-ring, can be solved. As for optical systems composed of a wide-angle lens group such as a car-mounted camera and a monitoring camera, most of the optical systems employ a negative lens or meniscus lens with a convex surface facing the object side as a front lens which is positioned at the forefront closest to the object side of the optical system. Particularly in the optical systems, it is easy to make a stepped portion where an O-ring is arranged on a circumference surface of the lens without enlarging the lens.
However, as downsizing of an image pickup unit is advanced in recent years, it becomes difficult to secure a large stepped portion where rubber gasket 60 is arranged on the circumference surface of the lens as shown in FIG. 5. Thus, arranging rubber gasket 60 properly on the stepped portion requires a technique. Further, if the arranged rubber gasket 60 is in the twisted condition, the rubber gasket 60 sometimes drops off its arrangement space at the time that distortion of the rubber gasket 60 is restored by its elastic force.
Further, when lens 10 is drawn out from case body 40 for the purpose of incorporating the lens 10 again, on the half way of the work to incorporate lens 10 in the case body 40 by pushing the lens in, it is possible to cause the following problem. If the rubber gasket 60 is pushed and is in contact already with an inner circumference surface of case body 40, only lens 10 is drawn out from the case body 40 and the rubber gasket 60 stays on the case body 40. Thus, the rubber gasket 60 sometimes drops off the lens 10.
When the rubber gasket 60 drops off the lens 10, it is necessary to mount rubber gasket 60 on the outer circumference surface of the lens 10 again, which results in occurrence of a problem that its incorporating work becomes complicated.
Further, the stepped portion is arranged on the rear side surface of the lens, and includes a surface perpendicular to the optical axis and a surface parallel to the optical axis. Therefore, harmful light to cause flare light tends to occur. When a wide-angle lens group employs this structure in particular, occurrence of harmful light becomes more remarkable, because harmful light further enters from an out of the angle of view easily.